Network Listening Mode (NLM) is a mode of operation during which an eNB listens to the downlink transmission of other eNBs. NLM is for instance used by small cells at start up in order to select a Physical Cell Identifier (PCI) that is not currently used in the area where the small cell is located (cf., for example, §22.3.5 in document [1]). NLM allows the small cell to read the broadcast channels of surrounding cells to determine which PCI are already used locally.
A Measurement Gap (MG) allows the UE to make inter-frequency and inter-RAT measurements. With the exception of the Random Access Channel (RACH) procedure that always has priority when started, no dedicated transmission/reception (TX/RX) activity takes place in the serving cell when a measurement gap occurs. A measurement gap is always 6 ms long in downlink (DL), 7 ms long in uplink (UL), and occurs with a periodicity of 40 or 80 ms.
The eNB can only use the receiver capability for NLM during time-slots with no transmissions from the eNB. While this is not an issue when a small cell is starting and has not yet any UE to serve, it is a major issue for scenarios requiring NLM to be used while having UEs already connected.